The present invention relates to voltage amplification circuits supplying a current at the output, and more particularly to amplification circuits designed to control the heads of the video tape recorders having various characteristics.
Some video tape recorders (VTRs) comprise four heads, two for a "normal duration" mode and two for a "long duration mode". Those four heads serve both for writing and reading. In operation, the two heads associated with the selected mode alternatively write or read data on a magnetic tape.
During recording, the voltage signals designed to be recorded are sent by a signal processor towards an amplification circuit supplying a current at the outputs connected to the VTR heads.
In such an amplification circuit, it is tried to accurately optimize and determine the transconductance which is the ratio between the output current and the input voltage for obtaining an optimum recording quality. It is also tried to accurately determine the input impedance for realizing a satisfactory impedance matching.
A single-head amplification circuit that meets those two requirements is mentioned in Digest of ICCF 1988 and is shown in FIG. 1. A coupling capacitor C is connected to the input terminal E of the circuit and transmits only high frequency signals. A differential amplifier D has a first input D1 connected to the coupling capacitor through a resistor R3, and its output D3 is connected to the base of an NPN transistor, the collector of which is connected to the output terminal S in turn connected to a head 1 of a VTR. An amplifier A has its input connected to the transistor emitter through a resistor R1 to ground through a protection resistor R, and its output connected to the second input D2 of the differential amplifier. A resistor R2 is connected between the input D1 of the differential amplifier and the transistor emitter.
The transconductance Tr of this circuit, in the event the gain of amplifier A is R3/(R2+R3), is expressed by: EQU Tr=(R1+R2)/(R1.times.R3) (1)
Values, R1=10.OMEGA., R2=1.OMEGA. and R3=1.5 k.OMEGA., for example, are chosen for optimizing this transconductance. Moreover, resistors R1, R2 and R3 are external and not integrated in order to be determined with a high accuracy as well as for optimizing the transconductance. For this circuit, the value of the protection resistor R which is integrated and determined with less accuracy has no effect on the transconductance value.
The input impedance of this circuit is equal to R3.
A method for realizing an amplification circuit permitting control of one of two heads having different characteristics is to provide two elementary circuits analogous to that of FIG. 1 having and common resistors R and R1. The differential amplifiers and transistors are identical in the two elementary circuits. Amplifier A becomes an amplifier A1 is one of the elementary circuits and an amplifier A2 in the other. Those amplifiers A1 and A2 have their inputs connected to each other. Each of the two circuit outputs is connected to a different head.
A switch K1, shown in dotted lines in FIG. 1, is added in each of the the elementary circuits. This switch is connected between the transistor base and ground and is constituted by a single transistor which is saturated when switched on.
Moreover, it is possible to add an external switch between the common coupling capacitor and the two resistor R3, but such a switch is expensive and not integrated. Another approach consists in replacing the external switch by a switch M shown in dotted lines between resistor R3 and the input D1 of the differential amplifier in each elementary circuit. This switch can be integrated but, since it has no gronded terminal, it has to be constituted by a circuit comprising several transistors (seven transistors).
In order to write through one of the heads, switch M is switched on and switch K1 of the circuit associated with the selected head is switched off, and the circuit switches that are to be rendered inactive are set to reverse position. Switching on a switch K1 connects the transistor base to the ground and current no longer flows through this transistor.
If two resistors R3 identical to the one of FIG. 1 are selected, the input impedance will still be equal to R3/2 when either elementary sub-circuit will be rendered inactive.
Considering in the elementary circuit comprising the amplifier A1 a resistor R2a and in the other elementary circuit a different resistor R2b instead of resistor R2, and if the gains of amplifiers A1 and A2 are chosen equal to R3/(R2a+R3) an R3/(R2B+R3), respectively, two R-independent transconductances are obtained: EQU Tra=(R1+R2a)/(R1.times.R3) EQU Trb=(R1+R2b)/(R1.times.R3)
However, providing an integrated switch M involves the provision of an additional pad for each elementary circuit, since resistors R2a, R2b and R3 are external. Thus, a pad between resistor R3 and the integrated circuit, and a pad between the integrated circuit and the terminal of resistor R2a (R2b) connected to the input D1 of the differential amplifier are to be provided. Without switch M, resistors R3 and R2 were connected to each other and a single pad was necessary for connecting them to input D1.
Thus, an object of the invention is to provide for an amplification circuit with a determined input impedance having two different transconductance values wherein no additional pad is added.
A further object of the invention is to provide such an amplification circuit comprising fewer transistors.